1. Field of the Invention
The present invention relates generally to skeletal plate systems for aligning and maintaining bone portions of the same bone or of different bones in a selected spatial relationship for healing or fusion of the bone portions, respectively. In particular, the present invention relates to skeletal plating systems comprising a plate that is flat and/or convex over a substantial portion of the lower surface of the plate along the longitudinal axis of the plate, bone screws, and locks for locking the bone screws to the plate; to segmentable plates; crossing screw plates; and combination bone screw-lock-plate systems permitting or causing, intersegmental bone compression and/or shortening.
2. Description of the Related Art
It is current practice in orthopedic surgery to use plating systems for joining portions of a broken bone, or for fusion of portions of separate bones. Such systems are composed essentially of plates and screws for aligning and holding the bone portions in a desired position relative to one another. Plating systems have usefulness in the spine, and have general skeletal use on the flat bones, such as the scapula and the pelvis by way of example, and for use on tubular bones, such as the humerus, ulna, radius, femur, and tibia by way of example.
Problems associated with such plating systems have included hardware breakage, hardware loosening, inability to gain adequate fixation, and distraction pseudoarthrosis where the plate will not allow the bone portions to come together over time resulting in a failure to get solid bone healing. These occurrences may cause problems, be associated with surgical failure, and require further surgical procedures to repair the damage, remove the failed hardware, and/or to reattempt skeletal stabilization.
Plates are usually provided to the surgeon for use in sets having a range of sizes so as to provide for such features as biological variability in size, the numbers of segments to be joined, and the length of the portions of bone to be joined. By way of example, it would be common for a plating system for use on the anterior cervical spine and for joining from two to five vertebrae to comprise of from forty to sixty plates. This requires manufacturers to make a large number of different plates, resulting in increased manufacturing costs and inventory costs and increased costs for hospitals to stock large numbers of plates. Further, in the event that a plate is used and another of its kind is needed before it can be replaced, the ability to provide to a patient the best care could be compromised.
Known plating systems additionally experience problems in connection with those procedures where bone grafts are placed between vertebral bodies to achieve an interbody fusion which heals by a process called xe2x80x9ccreeping substitutionxe2x80x9d. In this process, dead bone at the interfaces between the graft and the adjacent vertebra is removed by the body, as a prelude to the new growth of bone forming cells and the deposition of new bone. While the plates allow for proper alignment of the vertebrae and their rigid fixation, they can therefore, at the same time unfortunately, hold the vertebrae apart while the resorption phase of the creeping substitution process forms gaps in the bone at the fusion site with the result that the desired fusion does not occur. Such failure in an attempted fusion is known as pseudoarthrosis. A similar phenomenon occurs at the interface of a fractured bone""s fragments and is known as non-union. When such a failure occurs, the hardware itself will usually break or become loosened over time requiring further surgery to remove the broken hardware and to again attempt fusion or fracture repair.
Based on a consideration of the features of all of the known plating systems, there remains a need for an improved plating system having the following combination of features:
1) The plate and screws should be sufficiently strong to perform their intended function without mechanical failure;
2) The hardware, and specifically the screws, should be capable of achieving adequate purchase into the bone;
3) Means should be provided for locking each and every bone screw to the plate, and the locking means should be of sufficient size and strength to reliably perform its intended functions;
4) Bone screw locking means should preferably be retainable by the plate prior to bone screw insertion, or should be reliably attachable to a driver to prevent any small parts from becoming loose in the wound;
5) Bone screw orientation should be provided to create maximum screw purchase into bone and high resistance from being dislodged from the bone;
6) An improved and lower cost of production method for the manufacturer of medical plates should be provided;
7) A plate system should be provided for use in various sizes of patients which can be easily made to a selected length by a surgeon to fit the desired application in order to substantially reduce the number of plates required; and
8) Bone screw and plating system should be provided that prevent holding apart of bone portions during the process of creeping substitution and causes, or permits, or both causes and permits the bone portions to move toward each other to permit and promote the fusion or healing of the bone portions.
The present invention meets the above stated needs by providing various embodiments which are combinable, and may all be utilizable in the same plating system, such embodiments include (1) a skeletal plating system comprising a plate, that is flat over a substantial portion of its lower surface along the longitudinal axis of the plate and/or that has a lower surface that is convex curved along a substantial portion of the longitudinal axis of the plate, bone screws, and locks for locking the bone screws to the plate for skeletal use; (2) a skeletal plating system that permits a pair of bone screws to be inserted into a bone portion in a crossed over orientation and locked in place to the plate; (3) a segmentable skeletal plating system constructed so as to be selected for length by the surgeon; and (4) a combination screw-lock-plating system capable of allowing or urging bone portions together.
a. Multiple Lock System
The plating system of a first embodiment of the present invention comprises a general use skeletal plate having a bottom surface for placement against bone portions, wherein a substantial portion of the bottom surface of the plate is either flat or convex along the longitudinal axis of the plate. It is appreciated that a lesser portion of the lower surface of the plate may be otherwise shaped. The plate of the present invention has a plurality of bone screw receiving holes which extend through the plate, from the upper surface to the lower surface. The plate and its component parts, may be made of any implant quality material suitable for this purpose and suitable for use in the human body, such as, but not limited to, titanium or its alloys. The plate and/or the associated components may be made of a bioresorbable material and may comprise or be coated at least in part with fusion promoting chemical substances, such as bone morphogenetic proteins and the like.
Bone screws are each insertable into a respective bone screw receiving hole for attaching the plate to bone. A locking element, preferably, but not necessarily, in the form of a screw, is engageable in the locking screw hole of the plate and has a head formed to lock at least two of the bone screws to the plate. In the preferred embodiment, the locking elements are pre-installed prior to use by the surgeon in a manner so as to not impede installation of the bone screws into the bone screw receiving holes.
As a result, the problems previously associated with the locking screws of the type applied after the insertion of the bone screws, including the problems of instrumentation to position and deliver to the plate the locking means, backing out, breakage, stripping and misthreading associated with the prior art more delicate locking screws resembling xe2x80x9cwatchmaker""s partsxe2x80x9d, are eliminated.
b. Single-lock System
The plating system of the second embodiment of the present invention comprises a single-lock plate for skeletal use having a bottom surface for placement against bone portions, wherein a substantial portion-of the bottom surface of the plate is either flat or convex along the longitudinal axis of the plate. The single-lock plate has a locking element that fits within a bone screw receiving hole or into a recess overlapping a bone screw receiving hole to lock a respective one of the bone screws in place. According to this second embodiment of the invention, each of the bone screws is locked to the plate by means of an individual locking element which covers at least a portion of the bone screw. Since in the preferred embodiment of the single-lock plate, no other holes need be formed in the plate to attach the locking elements to the plate, the plate remains quite strong, or alternatively can be made thinner or narrower while keeping the requisite strength for the particular application.
The locking elements can be in many forms to achieve their intended purpose, such as, but not limited to, screws, threaded caps, rivets, set screws, projecting elements, and the like.
In common, neither the single-lock nor the multiple lock plating system requires that the head of the bone screw be hollow, as per some prior known plating systems. It will be appreciated that bone screws are weakened when their heads or head and neck portions are hollow so as to accommodate a second screw at least in part, if not wholly within.
In a further embodiment of the present invention, combinable in application with either the multiple lock or the single-lock systems and other novel features herein taught, a plate provides for the crossing over of the shafts of at least a pair of bone screws within a bone portion A crossed orientation of the screws within the bone provides a more secure engagement of the plate to the bone to which it is to be applied because longer screws may be used and because an area of bone is wedged and trapped between the screws as compared to plates which do not allow paired screws to cross. The use of further screws crossed and/or not crossed in combination with the crossed screw pair can be utilized to trap a still larger section of bone. The plate of the present invention may have multiple bone screw receiving bores (with fixed central longitudinal axes) in which the bores are oriented in a staggered configuration, such that the center points of each of the paired bone screw hole receiving bores are on different transverse lines to permit at least a pair of bone screws to be inserted in a crossed-over configuration within a bone portion. Preferably, the screw bores have defined longitudinal axes in the transverse plane of the plate though the screws may be capable of a variation in positioning as will subsequently be described. In the preferred embodiment, the included angle formed by the shafts of the crossed screws is between 25 to 90 degrees. For spinal use, by way of example, the paired screws are staggered, but are still alignable within the same vertebra so as to be diagonally crossed within that same vertebra and preferably crossed within the posterior two thirds of the vertebral body.
In a further embodiment of the present invention a segmentable plating system is disclosed combinable with the multiple lock and single-lock plating system and the crossing screw teaching, as well as combinable with other novel features herein taught. The segmentable plating system provides a single plate, or a limited set of plates, for aligning and maintaining bone portions in selected spatial relationship in which the plates are manufactured so as to be strong in use, but separable into shorter lengths by the surgeon as needed, thereby eliminating the need to stock a multitude of plate lengths.
By way of example, for application in the spine, an embodiment of the segmentable plating system of the present invention comprises a plate that is capable of spanning multiple segments of a cervical spine and has predetermined separation zones. The separation zones may be positioned in a segmentable plate such that when a portion of the segmentable plate would be applied to the vertebrae, the remaining separation zones in the plate, if any, would be supported by an underlying vertebrae. In use, the surgeon would determine the appropriate plate length needed and if the length needed was less than the length of the provided plate, the surgeon would remove the unneeded portion of the plate at the appropriate separation zone. By way of example, this procedure may be easily performed when the plate is made of titanium or one of its alloys, as the properties of titanium are such that when the plate is bent and then returned to its original position, a clean separation is made at the bend. The parts of the segmentable plates that are being separated can be held to either side of the separation zone to ensure that a precise separation is effected. The separation zones of the segmentable plate, by way of example, may comprise of the plate being scored along its upper, lower, or both upper and lower surfaces. The depth of such scores being dependent on the thickness of the plate, and being sufficient to create surface notchings and a path of least resistance for the plate separation, and yet of limited depth and shape, so as to not weaken the plate so as to render it less than sufficiently strong for its intended use.
By way of example, for application to the anterior aspect of the cervical spine four segmentable plates each having generally a similar length for example sufficient to span five vertebrae (a length of from 80 to 120 mm), and each having different spacings between pairs of bone screw holes could comprise a complete set of plates allowing a surgeon to have all lengths and hole spacings needed to fuse from two to five vertebrae. While the described plates may be separable into a multitude of usable portions, because of regulatory issues involving the identification of each implant with a distinct and singular implant identification number for tracking purposes it may be desirable to configure the plates of the present invention such that each plate will yield only one usable portion, such as is taught in the present invention.
The segmentable plating system of the present invention also has application in reconstructive surgery. For example, during repair of a broken eye socket, the segmentable plating system of the present invention can be used to align and maintain the broken bone portions in correct spatial relationship. The curved characteristic of an eye socket would require the plate used to repair the socket to match the curvature. The segmentable plate of the present invention may be made of a malleable metal, with the malleability of the plate being enhanced by the segmentation of the plate, such that it can more easily be contoured by the surgeon to the appropriate curvature. The correct length of the segmentable plate can also be easily obtained by the surgeon as already described. It should be noted that if for example surgical titanium alloy is selected for the plate material, then the separation zones allow the plate to be more easily bent, but without separating. The present invention makes a virtue of the material property of that alloy in that it may be bent without damage, but fails with surprisingly little force if first bent and then bent back. Back bending is therefore only done for plate separation and is not needed for contouring which requires only primary bending.
The ability to separate a plate into segments also provides significant advantages in the manufacturing process. By way of example, in the process of investment casting, a process commonly used to produce plates. The investment casting cost of material is minor relative to the labor involved in the casting process for the production of each plate regardless of size. It is far more economical to cast one eight inch long plate, which is later separable into four two inch long plates, than to make four two inch castings. If machining is included in production, as from bare stock or stamping or casting, that work can be automated, but the placing of the piece into the machine and securing it (fixturing) generally requires hands on attention, is time consuming, and is a potential manufacturing bottleneck. An eight inch long plate yielding four two inch plates potentially separable at the end by the machine doing the machining, may be fixtured only once. In contrast, the prior art method of manufacturing would require each of the four two inch long plates to be fixtured separately, one at a time. Therefore, the manufacturer can cast one long segmentable plate which can then be separated in the later manufacturing stages to yield multiple plates at an overall lower cost. Similarly, if the plate were in the alternative to be manufactured by machining from solid stock, great labor could be saved by fixturing and securing a single long plate that is later separable into multiple plates rather than having to fixture and secure each of those plates individually.
In a further alternative embodiment combinable with both the single-lock and multiple lock plate designs, the crossed screw teaching, and the segmentable plate teaching as well as other novel aspects of the present invention taught herein, three types of combination screw-lock-plate systems are taught, each capable of intersegmentable shortening and/or compression. Each of the taught systems is designed to counteract and compensate for the lack of contact between bone portions to be joined that may occur as a result of creeping substitution described above. The present invention will allow the vertebrae to move toward an interposed bone graft, and each other if necessary, instead of keeping the vertebrae apart during the occurrence of tile resorption phase of the creeping substitution process. Unlike prior art xe2x80x9cdynamicxe2x80x9d and/or compression plating systems, the present invention may allow for the preservation and/or enhancement of lordosis while otherwise restricting the motion of the bone screws relative to the plate.
The three types of screw-plate-lock systems, which are themselves combinable with one another, are as follows: (1) Passive Dynamic; (2) Self-Compressing; and (3) Active Dynamic and are described below.
a. Locked Passive Dynamic Plating System
As used in this description, the term xe2x80x9clockedxe2x80x9d means the screws are locked to the plate and can not backout. The term xe2x80x9cdynamicxe2x80x9d means the screw is capable of movement even though it is locked within the plate to allow bone portions to move closer together. The term xe2x80x9cpassivexe2x80x9d means motion of the screw relative to the plate is allowed, but not caused.
The passive dynamic system allows a bone screw to move relative to the plate even after being locked to the plate when a force is presented against the screw. This system does not cause screw movement, but only allows for movement of the screw to occur and thus is a xe2x80x9cpassivexe2x80x9d system. In a preferred embodiment, motion of the screw relative to the plate is confined to but one direction, that direction permitting bone portions to move closer to one another along the longitudinal axis of the plate.
In the passive dynamic system, a plate having a screw hole passing through the top and bottom surfaces of the plate for receiving a bone screw, may have a round opening at the top of the plate and may have a bottom opening that is oblong-shaped with a length greater than the diameter of a bone screw shaft locatable the screw hole when in use. The head of the bone screw is secured to the plate against backing out and generally against significant linear motion with a locking element, while the shaft of the bone screw is capable of angular motion relative to the plate. The oblong-shaped bottom opening of the screw hole allows the shaft of the bone screw to travel relative to the plate while the bone screw head rotates. The movement of the screw is greatest at the distal end of the screw, allowing for differential shortening of the bone portions being joined. For example, if such a plating system is applied to the anterior aspect of the cervical spine, lordosis (a convex curvature forward of the aligned vertebrae of the neck when viewed from the side) is enhanced when said passive movement occurs.
b. Self-Compressing Locking Plate System
In the self-compressing system, as a bone screw undergoes final tightening, or as it is being locked to the plate with a locking element the bone screw is forced to move in one allowed and desired direction. The bone screw can not move back once it is locked to the plate by the locking element. A purpose of the self-compressing system is to provide a fixed and locked angle of the bone screw relative to the plate for providing compression of bone portions to be joined, such as for example the cervical vertebrae adjacent a disc space, with movement of the bone screw as it is seated to the plate, producing compression and lordosis.
Unlike prior screw systems, the screws are only allowed to move in one direction, that being the direction that would bring bone portions to be joined closer together by angular motion, rather than to produce translational motion of a screw as a whole, without angular change. This induction of a compressive load across bone portions to be joined or fused, induces bone growth and when bone resorption occurs at the interface of the bone portions to be joined, those bone portions are urged to move closer together, thus avoiding the formation of a gap so as to mitigate against non-union or pseudoarthrosis.
The self-compressing system may comprise a plate having a bone screw receiving hole passing through the top and bottom surfaces of the plate with a top opening that is round and has a rounded seat. The bone screw receiving hole has bottom opening that has a central longitudinal axis that is offset from the central longitudinal axis of the top opening. The bone screw may have a partially rounded head which fits within the upper portion of the bone screw opening and permits movement of the screw head within the top opening in order to provide the appropriate angle for the bone screw shaft with respect to the plate as the bone screw shaft passes through the bottom opening.
Further it is known in the art that compressive forces across the bone further induce bone growth and formation and the present invention teaches novel ways of maintaining bone to bone contact, compressive loading, and even a means for enhancing and increasing the compressive load. A further benefit of the present invention can be appreciated by way of example in regard to use of the present invention on the anterior cervical spine for spinal fusion.
c. Active Dynamic Locking Plating System
In the active dynamic system, a pre-load force is applied to a bone screw such that while the screw may undergo no added motion initially, there is a selective force applied to the screwhead and the screw is capable of motion in only one direction, such that should resorption occur at the interfaces of the bone portions to be joined then the screw is not only free to move in that, and only that direction, but is also urged to do so as it moves to relieve the preload force. Features of these systems may be combined with each other.
By way of example only and not limitation, a plating system may utilize bone screw holes that have a lower surface opening that is oblong and extends from the center aligned to the longitudinal axis of the bone screw receiving bore in a direction for which screw motion is desired. A loading means such as a Bellville washer, lock washer, or other springing means is employed to bear upon the screw head when the screw is locked within the plate from backing out. Such a system urges the bone portions together over time as resorption permits.
For any given use, (plate, screw, hole, and spring) it is simple to determine correct resistance, that being an amount less than would break the bone to which the force is being applied. The Belville-type washer can have a tab which fits into a recess formed within the top opening of the screw hole in order to facilitate proper orientation of the washer or the washer or spring means can be other than round so as to be directionally orientable when placed within the top opening of the screw hole.
When features of these self compressing and active dynamic systems are combined, such a system forces bone portions close upon tightening and then both allows and urges such further motion, as resorption permits over time. The bone screw will only move further in the pre-oriented direction if there is space available and if there is an opposing force present less than he pre-loaded force on the screw.
It is an object of the present invention to provide an improved plating system which has the above described features and which avoids many of the shortcomings of previously known systems.
It is another object of the present invention to provide a locking mechanism where a plurality of bone screws used for attaching a plate to a bone portion can be easily and reliably locked in place at the same time by a single operation, and wherein the locking mechanisms for locking the bone screws may be pre-installed by the manufacturer prior to the insertion of the bone screws by the physician so that the physician does not have to attach the locking mechanism to the plate as a separate procedure during the operation.
A further object of the invention is to provide plates which are textured or otherwise treated to promote bone growth beneath the plate.
Yet another object of the invention is to provide a system in which the bone screws and locking mechanisms, when fully installed, have a low profile.
It is another object of the present invention to provide for a plating system which may be at least in part bioresorbable.
It is another object of the present invention to provide for a plating system comprising at least in part of bone ingrowth materials and surfaces.
It is another object of the present invention to provide for a plating system comprising at least in part of bone growth promoting substances.
It is another object of the present invention to provide plates with an improved holding ability within the bone due to a locked screw to plate crossover configuration.
It is another object of the present invention to provide a locked plating system capable of selected and specific screw motion so as to accommodate shortening of the bones to be joined.
It is another object of the present invention is to provide means for preventing distraction pseudoarthrosis of the anterior cervical spine, while providing for cervical lordosis.
The above and other objects and features of the invention will become more readily apparent from the following description of preferred embodiments of the invention, provided with reference to the accompanying drawings, which illustrate embodiments of the invention solely by way of non-limiting example.